Please do not post your data (the Base64-encoded string) in a question like this. It's completely unnecessary and simply makes your question difficult to read.
Are you writing a managed .NET COBOL application? ToBase64String is a method of the .NET Framework's System.Convert class - or at any rate there is a method by that name in that class.
If that's what you're trying to use, then you have the wrong method. ToBase64String converts arbitrary data to Base64. It does not convert Base64-encoded data to its original form.
For that you want System.Convert.FromBase64String. It has a single parameter, which is the string containing the Base64-encoded data, and it returns a byte array.